This invention relates to length measuring devices of the kind having metering means such as a measuring wheel which may be rotated by displacement of the device relatively to an article to be measured and a counter comprising a plurality of coaxial number wheels for counting revolutions of the measuring means.
A manually operated device of this kind typically comprises a housing which rotatably supports a series of number wheels connected by a suitable gear train to a measuring wheel, a portion of which projects from the housing. To measure a length along a surface, the device is held either in the hand or at the end of a handle and is moved across the surface with the measuring wheel rotated by frictional contact with the surface.
If a sensible measurement is to be obtained it is of course imperative that there should be no slip between the periphery of the measuring wheel and the surface, and sufficient pressure must therefore be applied to the device in a direction normal to the surface to achieve the necessary frictional forces; this pressure will of course vary with the coefficient of friction of the surface to be measured. It has been a danger with previous devices that if insufficient pressure were applied during a measurement, the user would be unaware that slip had occurred and would in error accept the reading of the number wheels as correct; this danger is particularly acute where the device has functioned correctly over a major part of the surface so that the incorrect reading appears to the user as reasonable.
Attempts have been made to overcome this problem of slip by reducing to an absolute minimum the resistance to rotational movement of the measuring wheel so as to reduce the amount of pressure that has to be applied by the user. As a consequence, however, there is a danger that accidental contact with the measuring wheel either at the beginning or the end of a measurement will cause a spurious rotation of the wheel and so introduce an error into the reading. In addition there may be a tendency for the measuring wheel to continue to rotate under its own inertia for a short while after the device has been lifted from the surface, again introducing an error into the reading.